1. Field of the Invention
The present invention relates to a dot matrix printer for effecting printing of an image on a printing medium according to print data received, by scanning a print head.
2. Description of the Related Art
The recent spread of desktop publishing has increased the needs to be met by high-functional printers to cope with a variety of fonts to be used and print formats. Some of page printers such as laser printers meeting the above needs have been developed. In general, however, the page printers are expensive, and so inexpensive high-functional printers are desired. As a kind of such inexpensive high-functional printers such as ink jet printers have remarkably been improved in print quality and density, and some of them having a print quality equal to or higher than that of the page printers have been developed. In view of such a trend, the development of the function to enable the use of several kinds of fonts and the printing with various formats is now necessary in not only the page printers but also the dot matrix printers.
The printing by such dot matrix printers is carried out in the following manner. That is, a character pattern corresponding to a character code transmitted from a host computer is read from a ROM previously storing it, and the character pattern is stored as a dot matrix image pattern into a buffer. Then, the dot matrix image pattern stored in the buffer is read to be applied to a print head, and is printed on a printing medium by the print head. The print head is mounted on a carriage. The carriage is moved in a direction perpendicular to a feed direction of the printing medium, thereby printing an image in a belt-shaped print area. After the printing of the image in the print area is ended, the printing medium is fed by a motor or the like.
FIGS. 5A and 5B illustrate print examples in the prior art printers. A print width in a dot matrix printer as mentioned above usually corresponds to a width of a print head. Accordingly, the simplest printing method in printing an image of one page is to feed a printing medium by the print width after printing one print area and then repeat this operation to obtain a continuous print of the image on the page. In the case of printing character strings at a line pitch larger than the width of the print area as shown in FIG. 5A, there is a possibility that some of the character strings printed lie on a boundary of adjacent print areas with a feed quantity of the printing medium being fixed, causing slippage of the printed image. More specifically, the character string "Mt. Fuji" on the second line is divided by the boundary between the second print area and the third print area. The upper and lower print areas adjacent to each other cannot be accurately jointed together along the boundary because of a problem in accuracy of the feed motor in feeding the printing medium and an influence of slip or play between the printing medium and a feed system, resulting in a slight gap or overlap between the adjacent print areas. Such a gap or overlap causes a reduction in quality of the printed image.
This problem is solved by skipping a blank portion between print lines in many printers and work processors. For example, as shown in FIG. 5B, the printing medium is fed by a quantity corresponding to a width of a blank portion between the adjacent character strings. As a result, the character string "Mt. Fuji" on the second line is not divided into two portions to be printed in the second and third print areas as in FIG. 5A, but is fully admitted within one print area.
FIGS. 6A to 6D are diagrams illustrating other print examples in the prior art printer, wherein two character strings each having a width smaller than the width of the print head (i.e., the print width) exist in one print area without vertically overlapping; and FIGS. 7A to 7D are diagrams illustrating still other print examples in the prior art printer, wherein two character strings including heading characters or script characters each having a width smaller than the print width exist in one print area with these strings being horizontally shifted and vertically overlapped at different horizontal print positions.
The printing method wherein an image pattern is printed per print area with a fixed feed quantity is not considered. In the case where two character strings are fully admitted within one print area as shown in FIGS. 6D and 7D, the two character strings can be printed without a cut by a single printing operation in the print area. To the contrary, in the case where two character strings are not fully admitted within one print area, but a part of any character, e.g., a lower part of a character "p" in the character string "Japan" projects out of the print area as shown in FIGS. 6A and 7A, the two character strings cannot be printed by a single printing operation in the print area, but the lower projecting part of the character only will be printed in the next printing operation in the next print area. As a result, slippage of the printed image as mentioned previously will occur.
Next, the printing method wherein character strings are printed per print line is considered. In the case of printing the two character strings as shown in FIGS. 6A and 7A, the upper character string "USA" is first printed as shown in FIGS. 6B and 7B. Then, the printing medium is fed by a quantity corresponding to a difference between a reference line for the upper character string and a reference line for the lower character string, and the lower character string "Japan" is then printed as shown in FIGS. 6C and 7C. According to this method, each character string is not divided in printing, and therefore no slippage of the printed image will occur. However, different print areas must be set according to different reference lines in printing. Accordingly, even when two character strings are fully admitted within one print area as shown in FIGS. 6D and 7D, the printing operation for one print area must be doubled, causing a reduction in print speed.
As another technique different from the above techniques, there is described in Japanese Patent Laid-open Publication No. 3-47766, for example, a printing method by scanning plural times in printing characters larger in height than the width of the print head. When any character in a character string has a height larger than the width of the print head, the print head must be scanned plural times in printing the character string. However, when the height of each character in the character string is not larger than the width of the print head, a single scanning of the print head is preferred to improve a print quality.